In linear approximation the small deviations from circular geodesic motion are described by two independent harmonic oscillations with eigenfrequencies called radial and vertical epicyclic frequencies. However, a more realistic description of the processes in the accretion disk requires to include further non-linear terms in the perturbation equations. They give account for different forces exerted in the accreting fluid such as pressure, viscosity, magnetic fields etc., and lead to the coupling of the two epicyclic modes. Typically such interaction between the eigenfrequencies is a prerequisite for the excitation of resonances in dynamical systems, which are realized when the system reaches suitable conditions. Indeed, analytical and numerical investigations of different models of accretion show that resonances are frequently present in the accretion disks and seem to be their intrinsic feature [1]}-[2]}.
